| 蒙古国博洛大型金矿区花岗岩SHRIMP锆石U-Pb测年及地质意义 |
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| 关键词:granite SHRIMP zircon U-Pb dating Sr-Nd isotope Boroo gold deposit Mongolia |
| 基金项目:国家科技支撑课题(编号: 2007BAB25B02)；地质调查项目(编号: 1212010911029) |
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| SHRIMP Zircon U-Pb Dating of Ore-bearing Granite in the Boroo Large-size Gold Deposit, Mongolia and Its Geological Significance |
| The SHRIMP U-Pb age dating results of zircons obtained for the first time from biotite granite of Boroo complex in the Boroo large-size gold orefield suggest that this region experienced the late Paleozoic tectono-magmatic activity. Two groups of ages were obtained, i.e., 441.9 ± 6.6Ma and 452.2 ± 3.9 Ma, indicating Late Ordovician. The major elements of Boroo biotite granite are characterized by high silica (SiO2=76.43~75.90%), rich potassium (K2O/Na2O=1.72~1.52), rich alkaline ((K2O+Na2O)=8.03~8.22%, alkali-degree rate AR=3.99~ 4.13)), low calcium (CaO=0.77%~0.97%), low magnesium (MgO=0.15%~0.16%), low phosphorus (P2O5= 0.02%~0.05%), and quasi-aluminous to weak peraluminous nature (Al2O3=12.39%~12.74%, and A/CNK= 1.02~1.05). Rittmann index σ=1.93~2.05, implying calc-alkaline nature. In comparison with the A-type granite, their average chemical compositions are similar to each other; nevertheless, the average chemical composition of Boroo biotite granite is obviously different from that of S-type granite and standard granite. In the K2O-SiO2 diagram, the rock samples fall into the range of high-K calc-alkaline series. Geochemical characteristics of rare earth elements are ΣREE = (160.56~91.28) × 10?6, LREE/HREE = 7.72~ 5.01, (La / Yb) N = 9.49~5.99, δEu = 0.19~0.41, and δCe = 0.82~0.79. REE distribution patterns demonstrate a significant negative Eu anomaly and a "swallow-type" shape. Except for P and Ti which are relatively slightly depleted relative to the primitive mantle, trace elements show enrichment in different degrees. Trace element spider diagrams exhibit the depletion of Nb, Ta, Sr, Ba, P, Ti, U, and the en-richment of Rb, Th, Pb. The right-inclination trend characteristics of spider diagram curves suggest that the content of trace elements and their compatibility are simultaneously decreasing and that the rocks have experienced crystallization differentiation of magmatic evolution. According to Zhang Qi’s (2008) granite classification, Boroo granite is of the Nanling type; in combination with REE and trace element spider diagrams, Boroo granite should be a product of post-orogenic extensional setting. (87Sr/86Sr)ⅰ of Boroo biotite granite = 0.70986 ~ 0.70891, (87Sr/86Sr)ⅰof modern oceanic basalts = 0.702 ~ 0.706, (87Sr/86Sr)ⅰ average value of conti-nental crust is 0.719, and (87Sr/86Sr)ⅰ value of Boroo granite is in the range of 0.706 ~ 0.719, indicating that the magma source of Boroo granite should have been mainly of crust-derivation, partly with contamination of man-tle-derived materials. (143Nd/144Nd)i=0.512138~0.512115, with an average of 0.512127, and εNd(t) values range from 1.2 to 1.4, with an average of 1.3, showing positive values with characteristics of mantle source. Nd isotope two-stage model ages T2DM = 1070~1094Ma, with an average of 1082Ma, indicating that the source rocks of Boroo granite complex might have been products of Tuva-Mongolia massif in the late period of Middle Proterozoic. In the εNd (t) -(87Sr/86Sr)ⅰ diagram, the Boroo granite falls in Quadrant I. All these features indicate that the source rocks of Boroo biotite granite are possessed of both continental crust-derivation and the involvement of man-tle-derived materials. Boroo granite was produced by crust-mantle interaction and the participation of new sub-stances derived from the mantle and formed in a post-orogenic environment. Regional extension led to crust thin-ning, mantle uplifting and asthenospheric upwelling. Decompression resulted in mantle melting and the formation of basalt magma, whereas basaltic underplating gave rise to lower crust melting and the formation of granite. The concentrations of such elements as gold, silver, copper and zinc in Boroo biotite granite are lower than their re-spective Clark values except for lead, whose concentration is slightly higher than the Clark value. The ore-forming age of the Boroo gold deposit should be early Jurassic, indicating that Boroo granite complex only served as ore-bearing wall rocks of the Boroo gold deposit and provided ore-bearing space. The formation of the Boroo gold deposit was irrelevant to the formation of Boroo granite complex. |
| HOU Wan-rong,NIE Feng-jun,JIANG Si-hong,BAI Da-ming,LIU Yan,YUN Fei,LIU Yi-fei.2010.SHRIMP Zircon U-Pb Dating of Ore-bearing Granite in the Boroo Large-size Gold Deposit, Mongolia and Its Geological Significance[J].Acta Geoscientica Sinica,31(3):331-342. |
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